Mechanistic studies on the B(C(6)F(5))(3) catalyzed allylstannation of aromatic aldehydes with ortho donor substituents

Synthesis of Allenes by Hydroalkylation of 1,3-Enynes with Ketones Enabled by Cooperative Catalysis

A method for the synthesis of allenes by the addition of ketones to 1,3-enynes by cooperative Pd(0)Senphos/B(C6F5)3/NR3 catalysis is described. A wide range of aryl- and aliphatic ketones undergo addition to various 1,3-enynes in high yields at room temperature. Mechanistic investigations revealed a rate-determining outer-sphere proton transfer mechanism, which was corroborated by DFT calculations.

Switching between X-Pyrano-, X-Furano-, and Anhydro-X-pyranoside Synthesis (X = C, N) under Lewis acid Catalyzed Conditions

A variety of C-glycosides can be obtained from the fluoroarylborane (B(C6F5)3) or silylium (R3Si+) catalyzed functionalization of 1-MeO- and per-TMS-sugars with TMS-X reagents. A one-step functionalization with a change as simple as the addition order and/or Lewis acid and TMS-X enables one to afford chiral synthons that are common (C-pyranosides), have few viable synthetic methods (C-furanosides), or are virtually unknown (anhydro-C-pyranosides), which mechanistically arise from whether a direct substitution, isomerization/substitution, or substitution/isomerization occurs, respectively.

Crystal structure of ortho-methoxy benzaldehyde, C8H8O2 – a second polymorph and deposition of 3D coordinates

C8H8O2, tetragonal, P43212 (no. 96), a = 10.9521(5) Å, c = 23.3014(17) Å, V = 2795.0(3) Å3, Z = 16, R gt (F) = 0.0317, wR ref (F 2) = 0.0883, T = 200 K.

Versatile Catalytic Hydrogenation Using A Simple Tin(IV) Lewis Acid

Despite the rapid development of frustrated Lewis pair (FLP) chemistry over the last ten years, its application in catalytic hydrogenations remains dependent on a narrow family of structurally similar early main-group Lewis acids (LAs), inevitably placing limitations on reactivity, sensitivity and substrate scope. Herein we describe the FLP-mediated H2 activation and catalytic hydrogenation activity of the alternative LA iPr3 SnOTf, which acts as a surrogate for the trialkylstannylium ion iPr3 Sn+ , and is rapidly and easily prepared from simple, inexpensive starting materials. This highly thermally robust LA is found to be competent in the hydrogenation of a number of different unsaturated functional groups (which is unique to date for main-group FLP LAs not based on boron), and also displays a remarkable tolerance to moisture.

B(C6F5)3 catalysed 1,6-conjugate allylation of para-quinone methides: expedient access to allyl diarylmethanes

An efficient protocol for the synthesis of unsymmetrical allyl diarylmethanes through a Lewis acid catalysed 1,6-conjugate addition of allylsilanes to para-quinone methides is described.

A one-pot allylation-hydrostannation sequence with recycling of the intermediate tin waste

A one-pot allylation and hydrostannation of alkynals where the tin byproduct formed in the first step of the reaction is recycled and used in the second step of the sequence is presented. Specifically, a BF3·OEt2-promoted allylstannation of the aldehyde moiety in the alkynal is followed by the introduction of polymethylhydrosiloxane (PMHS) and catalytic B(C6F5)3, which convert the tin byproduct of the allylation into Bu3SnH, which then hydrostannates the alkyne in the molecule. (119)Sn and (11)B NMR data suggest an organotin fluoride species is formed during the allylation step and involved in the tin recycling step.

Activation of alkynes with B(C₆F₅)₃--boron allylation reagents derived from propargyl esters

Novel allyl boron compounds are readily synthesized via rearrangement reactions between Lewis acidic B(C6F5)3 and propargyl esters. These reactions proceed through an initial cyclization followed by ring-opening and concurrent C6F5-group migration. In the absence of disubstitution adjacent to the ester oxygen atom, an allyl boron migration rearrangement leads to formal 1,3-carboboration products. These allyl boron compounds act as allylation reagents with aldehydes introducing both a C3-allyl fragment and a C6F5-unit as a single anti-diastereomer. In these reactions, B(C6F5)3 activates the alkynes, prompting the rearrangement processes and enabling installations of C6F5 and R-groups.

Photoinduced Polysiloxane Architectures from Spirosiloxane Precursors via Intramolecular Hydrosilylation

In this letter a method is described to synthesize new polysiloxane architectures by photoacid catalysis. An oxasilaspirocycle is designed that is able to undergo a photoacid generator catalyzed ring-opening reaction leading to either a homopolymer or to copolymers with hydroxyl-terminated polydimethylsiloxane. These polymers feature a defined amount of double bonds in the backbone, which is controlled by the ratio of oxasilaspirocycle to the comonomer. The former was prepared by trispentafluorophenylborane-catalyzed intramolecular hydrosilylation of a dialkenyloxysilane with an appropriate structural motif. The UV-initiated polymerization was characterized via in situ IR spectroscopic studies to determine the rate of reaction.

Dramatic effect of Lewis acids on the rhodium-catalyzed hydroboration of olefins

The addition of Lewis acids such as trispentafluoroboron as cocatalysts has been found to have a dramatic effect on the Rh-catalyzed hydroboration of olefins with pinacol borane. For example, aliphatic olefins do not react at all in noncoordinating solvents, but with the addition of 2% of B(C(6)F(5))(3), the reaction is complete in minutes. Similarly, the reaction of aromatic olefins with HBPin occurs slowly and nonselectively in the absence of B(C(6)F(5))(3), but is accelerated and occurs more selectively in its presence. Preliminary mechanistic studies suggest that the B(C(6)F(5))(3) needs to be present throughout the course of the reaction, not just at the initiation stage, and implicate this species, along with THF, in the heterolytic cleavage of the B-H bond of HBPin.

Heterolytic cleavage of disulfides by frustrated Lewis pairs

The addition of diphenyl disulfide (PhSSPh) to tBu(2)P(C(6)F(4))B(C(6)F(5))(2) (1) affords the zwitterionic phosphonium borate [tBu(2)P(SPh)(C(6)F(4))B(SPh)(C(6)F(5))(2)] (2), while the addition of a base or donor solvent to 2 effected the liberation of disulfide and the formation of [tBu(2)P(C(6)F(4))B(donor)(C(6)F(5))(2)]. The reaction of 1 with S(8) gave tBu(2)P(S)(C(6)F(4))B(C(6)F(5))(2) (3). In a similar fashion, the frustrated Lewis pair of tBu(3)P/B(C(6)F(5))(3) reacts with RSSR to give [tBu(3)P(SR)][(RS)B(C(6)F(5))(3)] (R = Ph (4), p-tolyl (5), iPr (6)). In contrast, the corresponding reaction of BnSSBn yields a 1:1:1 mixture of tBu(3)P horizontal lineS, Bn(2)S, and B(C(6)F(5))(3). Species 4 reacts with p-tolylSSp-tolyl to give a mixture of 4, 5, PhSSPh, and p-tolylSS p-tolyl, while treatment of 5 with PhSSPh afforded a similar mixture. To probe this, a crossover experiment between [tBu(3)P(SPh)][B(C(6)F(5))(4)] (7) and [NBu(4)][(p-tolylS)B(C(6)F(5))(3)] (9) was performed. The former species was prepared by a reaction of 4 with [Ph(3)C][B(C(6)F(5)) (4)], while cation exchange of [(Et(2)O)(2)Li( p-tolylS)B(C(6)F(5))(3)] (8) with [NBu(4)]Br gave 9. The reaction of compounds 7 and 9 gave a statistical mixture of the cations [tBu(3)P(SR)](+) and anions [(RS)B(C(6)F(5))(3)](-), R = Ph, Sp-tolyl. The mechanism of this exchange process was probed and is proposed to be an equilibrium involving disulfide and the frustrated Lewis pair. Crystallographic data are reported for compounds 4-8, and the natures of the P-S cations are examined via DFT calculations.

Oligomerization of electron-deficient vinyl monomers through an ate-complex mechanism: a new role for b(c(6) f(5) )(3) lewis Acid

The Lewis acid B(C(6) F(5) )(3) in combination with hydrosilanes exhibits remarkable activity in the oligomerization of sulfone- and phosphonate-based monomers. This process opens new routes to high-tech silicone-based materials, i.e., thermoplastic elastomers and heat-resistant polysiloxanes.

Tuning Lewis acidity using the reactivity of “frustrated Lewis pairs”: facile formation of phosphine-boranes and cationic phosphonium-boranes

The concept of "frustrated Lewis pairs" involves donor and acceptor sites in which steric congestion precludes Lewis acid-base adduct formation. In the case of sterically demanding phosphines and boranes, this lack of self-quenching prompts nucleophilic attack at a carbon para to B followed by fluoride transfer affording zwitterionic phosphonium borates [R(3)P(C(6)F(4))BF(C(6)F(5))(2)] and [R(2)PH(C(6)F(4))BF(C(6)F(5))(2)]. These can be easily transformed into the cationic phosphonium-boranes [R(3)P(C(6)F(4))B(C(6)F(5))(2)](+) and [R(2)PH(C(6)F(4))B(C(6)F(5))(2)](+) or into the neutral phosphino-boranes R(2)P(C(6)F(4))B(C(6)F(5))(2). This new reactivity provides a modular route to a family of boranes in which the steric features about the Lewis acidic center remains constant and yet the variation in substitution provides a facile avenue for the tuning of the Lewis acidity. Employing the Gutmann-Beckett and Childs methods for determining Lewis acid strength, it is demonstrated that the cationic boranes are much more Lewis acidic than B(C(6)F(5))(3), while the acidity of the phosphine-boranes is diminished.

Nonchelated Alkene and Alkyne Complexes of d0 Zirconocene Pentafluorophenyl Cations

This paper describes the generation and properties of nonchelated d(0) zirconocene-aryl-alkene and alkyne adducts that are stabilized by the presence of beta-SiMe(3) substituents on the substrates and the weak nucleophilicity of the -C(6)F(5) ligand. The cationic complexes [(C(5)H(4)R)(2)Zr(C(6)F(5))][B(C(6)F(5))(4)] (4a: R = H, 4b: R = Me) were generated by methide abstraction from (C(5)H(4)R)(2)Zr(C(6)F(5))Me by Ph(3)C(+). NMR studies show that 4a,b contain an o-CF...Zr dative interaction and probably coordinate a PhCl molecule in PhCl solution. Addition of allyltrimethylsilane (ATMS) to 4a,b in C(6)D(5)Cl solution at low temperature produces an equilibrium mixture of (C(5)H(4)R)(2)Zr(C(6)F(5))(H(2)C=CHCH(2)SiMe(3))(+) (7a,b), 4a,b, and free ATMS. Similarly, addition of propargyltrimethylsilane (PTMS) to 4a produces an equilibrium mixture of Cp(2)Zr(C(6)F(5))(HCCCH(2)SiMe(3))(+) (8a), 4a, and free PTMS. The NMR data for 7a,b,and 8a are consistent with highly unsymmetrical substrate coordination and substantial polarization of the substrate multiple bond with significant positive charge buildup at C(int) and negative charge buildup at C(term). PTMS binds to 4a more strongly than ATMS does. The ATMS adducts undergo nondissociative alkene face exchange ("alkene flipping"), i.e., exchange of the (C(5)H(4)R)(2)Zr(C(6)F(5))(+) unit between the two alkene enantiofaces without decomplexation of the alkene, on the NMR time scale.

Nonchelated d0 Zirconium−Alkoxide−Alkene Complexes

The reaction of Cp'2Zr(O(t)Bu)Me (Cp' = C5H4Me) and [Ph3C][B(C6F5)4] yields the base-free complex [Cp'2Zr(O(t)Bu)][B(C6F5)4] (6), which exists as Cp'2Zr(O(t)Bu)(ClR)+ halocarbon adducts in CD2Cl2 or C6D5Cl solution. Addition of alkenes to 6 in CD2Cl2 solution at low temperature gives equilibrium mixtures of Cp'2Zr(O(t)Bu)(alkene)+ (12a-l), 6, and free alkene. The NMR data for 12a-l are consistent with unsymmetrical alkene bonding and polarization of the alkene C=C bond with positive charge buildup at C(int) and negative charge buildup at C(term). These features arise due to the lack of d-pi* back-bonding. Equilibrium constants for alkene coordination to 6 in CD2Cl2 at -89 degrees C, K(eq) = [12][6](-1)[alkene](-1), vary in the order: vinylferrocene (4800 M(-1)) >> ethylene (7.0) approximately alpha-olefins > cis-2-butene (2.2) > trans-2-butene (<0.1). Alkene coordination is inhibited by sterically bulky substituents on the alkene but is greatly enhanced by electron-donating groups and the beta-Si effect. Compounds 12a-l undergo two dynamic processes: reversible alkene decomplexation via associative substitution of a CD2Cl2 molecule, and rapid rotation of the alkene around the metal-(alkene centroid) axis.

Pentafluorophenyl Transfer: A New Group-Transfer Reaction in Organoborate Salts

[reaction: see text] Irradiation of isoquinolinium hydroxytris(pentafluorophenyl)borate, 1, and phenanthridium hydroxytris(pentafluorophenyl)borate, 2, in either CH2Cl2 or CH3CN resulted in C6F5 transfer to the isoquinolinium and phenanthridium cations, generating 2-methyl-1-(2,3,4,5,6-pentafluorophenyl)-1,2-dihydroisoquinoline, 3, and 2-methyl-1-(2,3,4,5,6-pentafluorophenyl)-1,2-dihydrophenanthridine, 4, respectively. In addition, photogeneration of H2O x B(C6F5)3 resulted from 1. Photogeneration of C6F5-C6F4H from HO-B(C6F5)3(-) and of C6H5-C6F4H from C6H5-B(C6F5)3(-) was discovered.

Tris(pentafluorophenyl)borane: a special boron Lewis acid for special reactions

Tris(pentafluorophenyl)borane is best known for its role as an excellent activator component in homogeneous Ziegler-Natta chemistry. However, the special properties of B(C6F5)3 have made this strong boron Lewis acid an increasingly used catalyst or stoichiometric reagent in organic and organometallic chemistry. This includes catalytic hydrometallation reactions, alkylations and catalyzed aldol-type reactions. B(C6F5)3 catalyzes tautomerizations and can sometimes stabilize less favoured tautomeric forms by adduct formation. It induces some rather unusual reactions of early metal acetylide complexes and can help in stabilizing uncommon coordination geometries of carbon. The growing number of such examples indicates an increasing application potential of the useful Lewis acid B(C6F5)3 aside from its established role in olefin polymerization catalysis.

Metal Cation−Methyl Interactions in CB11Me12- Salts of Me3Ge+, Me3Sn+, and Me3Pb+

Oxidation of Me(6)M(2) (M = Ge, Sn) and Me(4)Pb with the CB(11)Me(12)(*) radical in alkane solvents produced the insoluble salts Me(3)M(+)CB(11)Me(12)(-), characterized by CP-MAS NMR and EXAFS. The cations interact with methyl groups of CB(11)Me(12)(-) with coordination strength increasing from Pb to Ge. Density functional theory (DFT) calculations for the isolated ion pairs, Me(3)M(+)CB(11)Me(12)(-) (M = Ge, Sn), revealed three isomers with the cation above methyl 2, 7, or 12, and not above a BB edge or a BBB triangle. The interaction has a considerable covalent component, with the cation attempting to perform a backside S(E)2 substitution on the methyl carbon. In a fourth less favorable isomer the cation is near methyl 1, inclined toward methyl 2, and interacts with hydrogens. DFT atomic charge distributions and plots of the electrostatic potential on the surface of spheres centered at the CB(11)H(12)(-) and CB(11)Me(12)(-) icosahedra display the effects of uneven charge distribution within the anion and contradict the common belief that the negative charge of the cage anion is concentrated primarily on the cage boron atoms 7-12; in CB(11)Me(12)(-), roughly half is on the cage carbon and the rest on methyls 7-12.

Me2AlCH2PMe2: A New, Bifunctional Cocatalyst for the Ni(II)-Catalyzed Oligomerization of PhSiH3

The role of methylaluminoxane (MAO) in the Ni-catalyzed dehydrogenative homologation of PhSiH3 has been investigated with a view to designing new cocatalysts possessing well-defined chemical compositions and structures. These studies show that species such as the bifunctional reagent (Me2PCH2AlMe2)2, 3, should act as co-catalyst for the Si-Si bond formation reactions. Thus, it was found that the combination of (1-Me-indenyl)Ni(PPh3)Me, 2a, and 3 (Ni/Al ratio of 1:1) converts PhSiH3 to cyclic oligomers (PhSiH)n with a turnover frequency (TOF) of >500 h(-1), 50 times faster than with 2a alone. Detailed NMR studies have indicated that this acceleration is due to the formation of the intermediate (1-Me-indenyl)Ni(Me)(Me2PCH2AlMe2), 4. Coordination of the PMe2 moiety in this complex to the Ni center allows the tethered AlMe2 moiety to interact with the Ni-Me moiety in such a way that promotes fairly slow Al-Me/Ni-CD3 exchange (t(1/2) ca. 12 h) but accelerates the Si-H bond activation and Si-Si bond formation reactions. The catalysis promoted by 2a/3 proceeds even faster in the presence of NEt3 or THF (TOF > 1600 h(-1)), because these Lewis bases favor the monomeric form of 3, which in turn favors the formation of 4. On the other hand, the much more nucleophilic base quinuclidine suppresses the catalysis (TOF < 300 h(-1)) by hindering the Ni.R.Al interactions. These observations point to an emerging strategy for using bifunctional reagents such as 3 to place geometrically constrained Lewis acid moieties adjacent to metal centers, thereby activating certain metal-ligand bonds.

Weaker Lewis acid, better catalytic activity: dual mechanisms in perfluoroarylborane-catalyzed allylstannation reactions

[reaction: see text] PhB(C(6)F(5))(2) exhibits much higher activity as a Lewis acid catalyst for the allylstannation of aromatic aldehydes than the stronger Lewis acid B(C(6)F(5))(3). This anomalous enhancement of catalytic activity for the weaker LA is shown to be partly due to decreased thermodynamic stability of ion pair 2b relative to 2a in the product-forming step of the reaction. A mechanistic path where the borane serves as the true LA catalyst is more important for the weakly Lewis acidic borane.

Ring-opening reactions of nonactivated aziridines catalyzed by tris(pentafluorophenyl)borane

The ring-opening reactions of nonactivated aziridines with amine nucleophiles are efficiently catalyzed by tris(pentafluorophenyl)borane leading to derivatives of trans-1,2-diamines in high yields. A mechanistic investigation of the reaction suggests that in situ formed [(C(6)F(5))(3)B(OH(2))].H(2)O catalyzes the opening through a Brønsted acid manifold.

Norbornyl cations of group 14 elements

Norbornyl cations of the group 14 elements Si --> Pb have been synthesized from substituted 3-cyclopentenemethyl precursors by intramolecular addition of transient cations to the C=C double bond of the 3-cyclopentenemethyl substituent (pi-route to norbornyl cations). The norbornyl cations 4a (E = Si, R = Me), 4e (E = Si, R = Et), 4f (E = Si, R = Bu), 4g (E = Ge, R = Bu), 4h (E = Sn, R = Bu), and 4i (E = Pb, R = Et) have been identified by their characteristic NMR chemical shifts (4a,e,f, delta((29)Si) = 80-87, delta((13)C)(CH=) = 149.6-150.6; 4g, delta((13)C)(CH=) = 144.8; 4h, delta((119)Sn) = 334, delta((13)C)(CH=) = 141.5; 4i, delta((207)Pb) = 1049, delta((13)C)(CH=) = 138). The significant deshielding of the vinylic carbon atoms (Deltadelta((13)C)) relative to those of the precursor (Deltadelta((13)C) = 19.3-20.3 (4a,e,f), Deltadelta((13)C) = 14.6 (4g), Deltadelta((13)C) = 11.1 (4h), Deltadelta((13)C) approximately 8 (4i)) and the small J coupling constants between the element and the remote vinyl carbons in the case of 4h and 4i (J(CSn) = 26 Hz, J(CPb) = 16 Hz) give experimental evidence for the intramolecular interaction and the charge transfer between the positively charged element and the remote C=C double bond. The experimental results are supported by quantum mechanical calculations of structures, energies, and magnetic properties for the norbornyl cations 4a,b (E = Ge, R = Me), 4c (E = Sn, R = Me), 4d (E = Pb, R = Me), and 4e,f at the GIAO/B3LYP/6-311G(3d,p)//MP2/6-311G(d,p) (Si, Ge, C, H), SDD (Sn, Pb) level of theory. The calculated (29)Si NMR chemical shifts for the silanorbornyl cations 4a,e,f (delta((29)Si) = 77-93) agree well with experiment, and the calculated structures of the cations 4a-f reveal their bridged norbornyl cation nature and suggest also for the experimentally observed species 4a,e-i a formally 3 + 1 coordination for the element atom with the extra coordination provided by the C=C double bond. This places five carbon atoms in the close vicinity of the positively charged element atom. The group 14 element norbornyl cations 4a,e-i exhibit only negligible interactions with the aromatic solvent, and they are, depending on the nature of the element group, stable at room temperature in aromatic solvents for periods ranging from a few hours to days. In acetonitrile solution, the intramolecular interaction in the norbornyl cations 4a,e-h breaks down and nitrilium ions with the element in a tetrahedral environment are formed. In contrast, reaction of acetonitrile with the plumbyl cation 4i forms an acetonitrile complex, 10i, in which the norbornyl cation structure is preserved. The X-ray structure of 10i reveals a trigonal bipyramidal environment for the lead atom with the C=C double bond of the cyclopentenemethyl ligand and the nitrogen atom of the acetonitrile molecule in apical positions. Density functional calculations at the B3LYP/6-311G(2d,p)//(B3LYP/6-31G(d) (C, H), SDD (Si, Ge, Sn, Pb)) + DeltaZPVE level indicate that the thermodynamic stability of the group 14 norbornyl cations increases from Si to Pb. This results in a relative stabilization for the plumbanorbornyl cation 4d compared to tert-butyl cation of 52.7 kcal mol(-)(1). In contrast, the intramolecular stabilization energy E(A) of the norbornyl cations 4a-d decreases, suggesting reduced interaction between the C=C double bond and the electron-deficient element center in the plumbacation compared to the silacations. This points to a reduced electrophilicity of the plumbacation compared to its predecessors.